The present invention relates to a polymer electrolyte fuel cell to be used for co-generation systems and mobile electric power generators for e.g. automobiles or the like, particularly to a gas diffusion layer to be used for such fuel cells.
In a polymer electrolyte fuel cell, a fuel gas such as hydrogen and an oxidant gas such as air are supplied to a pair of gas diffusion electrodes (anode for the fuel gas and cathode for the oxidant gas), and are electrochemically reacted with each other at catalyst layers of e.g. platinum therein. Such reaction generates electricity and heat at the same time. A general structure of such polymer electrolyte fuel cell is shown in FIG. 1.
Referring to FIG. 1, opposite main surfaces of a polymer electrolyte membrane 11 for selective transporting hydrogen ions are provided with a pair of catalyst layers 12 to intimately contact the membrane respectively. Each catalyst layer 12 has a carbon powder as a main component and carrying platinum metal catalyst. Outside the respective catalyst layers 12 are provided a pair of gas diffusion layers to intimately contact the catalyst layers 12, wherein each gas diffusion layer 13 comprises a porous supporting carbon body, made of porous carbon material and supporting e.g. the catalyst layer. A combination of each gas diffusion layer 13 and each catalyst layer 12 constitutes a gas diffusion electrode 14.
A pair of separator plates 17 are provided outside the pair of gas diffusion electrodes 14: which mechanically fix a polymer electrolyte membrane-electrode assembly (MEA hereafter) 15 constituted by the gas diffusion electrodes 14 and the polymer electrolyte membrane 11; which electrically connect neighboring MEAs in series; and which have gas flow channels 16 at the surfaces thereof contacting gas diffusion electrodes for supplying the reactive gases to the gas diffusion electrodes, and for exhausting, to outside, water generated by the electrochemical reaction and excessive gases. The gas flow channels can be provided separately from the separator plates 17, but it is a general manner to provide grooves, as gas flow channels, on the surfaces of the separator plates. Further, gaskets 18 are provided and sandwiched between the polymer electrolyte membrane 11 and the separator plates 17 for preventing the reactive gases from leaking to outside.
During the operation of the fuel cell, the air or oxygen, which is an active reactive material, is diffused at the cathode side to the catalyst layer from the gas flow channels through the gas diffusion layer. At the same time, excessive water, together with excessive gases, generated by the electrochemical reaction and permeated to the gas diffusion layer from the catalyst layer on the basis of the osmotic effect is exhausted to outside of the fuel cell through pores of the gas diffusion layers.
The polymer electrolyte used in a polymer electrolyte fuel cell can maintain a necessary level of ionic or protonic conductivity when the electrolyte is put under a sufficiently wet condition. So generally, the reactive gases are preliminarily humidified to a given humidity, thereby to secure the humidity of the polymer electrolyte membrane as well as the supply of the reactive gases.
On the other hand, the electrode reaction in a cell is a water generation reaction caused by the three-phase interfaces of the catalyst, the polymer electrolyte and the reactive gases. Accordingly, if the supplied water vapor and the water generated by the electrode reaction are not quickly exhausted to outside, the gas diffusion electrodes or the gas diffusion layers suffer from water undesirably retained therein and get worse in the gas diffusion property, whereby the cell characteristics gets deteriorated.
In view of the foregoing, several countermeasures were taken for improving both the water retaining property and the water exhaustion property of the gas diffusion electrodes to be used for polymer electrolyte fuel cells. A general gas diffusion electrode uses a porous supporting carbon body, to be a gas diffusion layer, having formed thereon a layer of carbon powder, i.e. carbon particles, carrying a noble metal as a catalyst layer. Usually, one carbon material selected from carbon cloths and carbon unwoven fabrics such as carbon paper is used for a porous supporting carbon body. Generally, such porous supporting body is preliminarily subjected to a water repellent treatment by using e.g. a liquid dispersion of polytetrafluoroethylene (PTFE hereafter) material, which is a fluorocarbon material, so as to enable quick exhaustion of water generated during the electrode reaction, and to maintain the polymer electrolyte membrane and the polymer electrolyte in the gas diffusion electrode at an appropriate wet condition. Another countermeasure taken is to mix carbon particles, having been subjected to water repellent treatment, with the electrode catalyst layers, thereby to more readily exhaust excessive generated water in the electrode catalyst layers.
However, such conventional countermeasures are not enough. It is difficult according thereto, with respect to the whole region of each gas diffusion layer from an inlet side to an outlet side of each gas flow channel, to realize [I] a good balance between water retention and water exhaustion in the thickness direction of each gas diffusion layer, i.e. direction perpendicular to the plane of the gas flow channels or of the surface of the gas diffusion layer; and [II] a uniform water retention at gas diffusion layer from the inlet side to the outlet side of each gas flow channel.
Problems according to such prior art will be described below in more detail.
[I] Water Exhaustion and Water Retention in Thickness Direction of Gas Diffusion Layer
In a conventional polymer electrolyte fuel cell, a porous supporting carbon body, for a gas diffusion layer, is used for a gas diffusion electrode as described above. For such porous supporting carbon body, a carbon cloth or a carbon unwoven fabric such as a carbon paper is used. Generally, a carbon unwoven fabric has an isotropic gas permeability, whereas a carbon cloth has a higher gas permeability in its thickness direction than in its surface plane direction, because the carbon cloth, which is made of a mesh, has pores defined by the mesh. For this reason, a carbon cloth is generally superior to a carbon unwoven fabric in the function of exhausting excessive water generated at the catalyst layer electrically conductive, whereas the carbon unwoven fabric is superior to the carbon cloth in the function of retaining water therein.
Thus, the water exhaustion property and the water retention property in a gas diffusion layer are in a trade-off relation, i.e. incompatible, with each other. Therefore, it has been attempted to select an optimum material for the porous supporting carbon body from among various materials therefor, depending upon uses, namely upon specific operation conditions.
Therefore, when e.g. discharging currents vary, or when flow rates or amounts of humidification of the supplied gases change, then either amounts of water in the catalyst layers become short, or excessive water blocks the supplied gases from reaching the polymer electrolyte membranes, thereby to deteriorate resultant cell characteristics. Japanese Laid-open Patent Publications Hei 8-124583 and Hei 6-262562 describe a technology to gradually increase coarseness of the mesh of a carbon cloth from an inlet side to an outlet side of the gas flow channel, namely from a finer mesh at the inlet side to a coarser mesh at the outlet side thereof. Such technology cannot solve the problem regarding the water exhaustion property in the thickness direction of the gas diffusion layer.
It is necessary furthermore to allow the supplied gases to sufficiently reach the polymer electrolyte membranes, with the balance between the water exhaustion property and the water retention property being well maintained. In other words, high performance gas diffusion electrodes are needed to be designed in such a manner that water generated at the catalyst layers are quickly sucked out to the gas diffusion layers, and is then evaporated in the gas diffusion layers to be effectively exhausted to outside. Thereby, excessive water is not retained in the electrode catalyst, and the polymer electrolyte is kept at an appropriate wet condition, with the supplied gases sufficiently reaching the polymer electrolyte membrane.
Furthermore, it is difficult and is a factor of cost increase to manufacture a carbon cloth having a mesh varying its fineness or coarseness thereof gradually in the surface plane direction thereof.
[II] Uniform Water Retention at Gas Diffusion Layer from Inlet Side to Outlet Side of Gas Flow Channel
A part of water generated by the electrode reaction is flown, together with the reactive gases flowing in the gas flow channels of the separator plates, to the outlet of the gas flow channels, and is exhausted to outside of the fuel cell. Accordingly, the amounts of water contained in the reactive gases so vary in the flow direction of the reactive gases as to cause a larger amount of water at the outlet side than at the inlet side of the reactive gases due to water generated by electrode catalyst reaction or electrochemical reaction. This is likely to cause the amount of water at the outlet side to exceed a given threshold level, thereby to bring the outlet side to an excessively wet condition. For this reason, the fuel cell is likely to have a deteriorated water exhausting function at the outlet side of the gas flow channels. In an extreme case, a serious problem may arise in that the pores of the gas diffusion layers are occluded by the excessive water (flooding or flooding phenomena hereafter), whereby the reactive gases are so inhibited from necessary diffusion as to cause an extreme decrease of the cell voltage.
In contrast to the above, if the reactive gases to be supplied to the inlets of the gas flow are preliminarily so humidified with an amount of humidification as to prevent the flooding phenomena at the outlets thereof, then the water content in the polymer electrolyte membrane at or in the vicinity of the inlets of the gas flow channels is likely to excessively decrease. This causes a problem to decrease the protonic conductivity or increase protonic resistance of the polymer electrolyte membrane there, whereby the cell voltage very much decreases. A fuel cell having gas diffusion electrodes of larger area and gas flow channels of longer length suffers very much more from above described undesired characteristics.
A proposal for solving such problems is described in the Japanese Laid-open Patent Publication Hei 6-167562 as described above. The structure of a fuel cell described in this prior art is to increase the porosity of the gas diffusion layer from the inlet side to the outlet side of each gas flow channel as described also above. However, such prior art structure is likely to have problems of deteriorated basic performances of fuel cells in that the amount of gas diffusion gets non-uniform in the cell surface, and that gas diffusion electrodes get to have decreased electric conductivities at the outlet side of the gas flow channels or to have non-uniform electric conductivities in the cell surface.